MEMS device with continuous looped insert and trench

ABSTRACT

The present invention provides a MEMS device such as a capacitive MEMS microphone that comprises a new design of air flow restrictor having a pair of continuous looped insert and trench. An air channel/space includes a first internal wall and a second internal wall for air to flow between. A continuous looped trench is recessed from the first internal wall, and a continuous looped insert is extended from the second internal wall and inserted into the trench. The spatial relationship between the insert and the trench can vary or oscillate. Air resistance of the channel/space may be controlled by the trench&#39;s depth. The invention has a significant effect on, for example, keeping the sound frequency response plot more flat on the low frequency part ranging from 20 Hz to 1000 Hz.

CROSS-REFERENCE TO RELATED U.S. APPLICATIONS

This application is a Continuation-in-Part of U.S. non-provisionalapplication Ser. No. 16/000,860, filed on Jun. 5, 2018, which is aContinuation-in-Part of U.S. non-provisional application Ser. No.15/393,831 filed on Dec. 29, 2016 and granted as U.S. Pat. No.10,171,917 on Jan. 1, 2019, which two prior applications areincorporated herein in their entirety by reference.

FIELD OF THE INVENTION

The present invention generally relates to a MEMS device, such as alateral mode capacitive microphone, that includes a pair of continuouslooped insert and trench used as an air flow restrictor. The microphoneof the invention may find applications in smart phones, telephones,hearing aids, public address systems for concert halls and publicevents, motion picture production, live and recorded audio engineering,two-way radios, megaphones, radio and television broadcasting, and incomputers for recording voice, speech recognition, VoIP, and fornon-acoustic purposes such as ultrasonic sensors or knock sensors, amongothers.

BACKGROUND OF THE INVENTION

A microelectromechanical system (MEMS) is a microscopic device withmoving parts that is fabricated in the same general manner as integratedcircuits. For example, a MEMS microphone is a transducer that convertssound into an electrical signal. Among different designs of microphone,a capacitive microphone or a condenser microphone is conventionallyconstructed employing the so-called “parallel-plate” capacitive design.Unlike other microphone types that require the sound wave to do morework, only a very small mass in capacitive microphones needs be moved bythe incident sound wave. Capacitive microphones generally produce ahigh-quality audio signal and are now the popular choice in consumerelectronics, laboratory and recording studio applications, ranging fromtelephone transmitters through inexpensive karaoke microphones tohigh-fidelity recording microphones.

FIG. 1A is a schematic diagram of parallel capacitive microphone in theprior art. Two thin layers 101 and 102 are placed closely in almostparallel. One of them is fixed backplate 101, and the other one ismovable/deflectable membrane/diaphragm 102, which can be moved or drivenby sound pressure. Diaphragm 102 acts as one plate of a capacitor, andthe vibrations thereof produce changes in the distance between twolayers 101 and 102, and changes in the mutual capacitance therebetween.

“Squeeze film” and “squeezed film” refer to a type of hydraulic orpneumatic damper for damping vibratory motion of a moving component withrespect to a fixed component. Squeezed film damping occurs when themoving component is moving perpendicular and in close proximity to thesurface of the fixed component (e.g., between approximately 2 and 50micrometers). The squeezed film effect results from compressing andexpanding the fluid (e.g., a gas or liquid) trapped in the space betweenthe moving plate and the solid surface. The fluid has a high resistance,and damps the motion of the moving component as the fluid flows throughthe space between the moving plate and the solid surface.

In capacitive microphones as shown in FIG. 1A, squeeze film dampingoccurs when two layers 101 and 102 are in close proximity to each otherwith air disposed between them. The layers 101 and 102 are positioned soclose together (e.g. within 5 μm) that air can be “squeezed” and“stretched” to slow movement of membrane/diaphragm 101. As the gapbetween layers 101 and 102 shrinks, air must flow out of that region.The flow viscosity of air, therefore, gives rise to a force that resiststhe motion of moving membrane/diaphragm 101. Squeeze film damping issignificant when membrane/diaphragm 101 has a large surface area to gaplength ratio. Such squeeze film damping between the two layers 101 and102 becomes a mechanical noise source, which is the dominating factoramong all noise sources in the entire microphone structure.

Perforation of backplate has been employed to control the squeeze filmdamping to a desired range. For example, US Patent Application2014/0299948 by Wang et al. discloses a silicon based MEMS microphone asshown in FIG. 1B. Microphone 10 may receive an acoustic signal andtransform the received acoustic signal into an electrical signal for thesubsequent processing and output. Microphone 10 includes a siliconsubstrate 100 and an acoustic sensing part 11 supported on the siliconsubstrate 100 with an isolating oxide layer 120 sandwiched in between.The acoustic sensing part 11 of the microphone 10 may include at least:a conductive and compliant diaphragm 200, a perforated backplate 400,and an air gap 150. The diaphragm 200 is formed with a part of a silicondevice layer such as the top-silicon film on a silicon-on-insulator(SOI) wafer or with polycrystalline silicon (Poly-Si) membrane through adeposition process. The perforated backplate 400 is located above thediaphragm 200, and formed with CMOS passivation layers with a metallayer 400 b imbedded therein which serves as an electrode plate of thebackplate 400. The air gap 150 is formed between the diaphragm 200 andthe backplate 400. The conductive and compliant diaphragm 200 serves asa vibration membrane which vibrates in response to an external acousticwave reaching the diaphragm 200 from the outside, as well as anelectrode. The backplate 400 provides another electrode of the acousticsensing part 11, and has a plurality of through holes 430 formedthereon, which are used for air ventilation so as to reduce air dampingthat the diaphragm 200 will encounter when starts vibrating. Therefore,the diaphragm 200 is used as an electrode plate to form a variablecondenser 1000 with the electrode plate of the backplate 400. Theacoustic sensing part 11 of the microphone 10 may further include aninterconnection column 600 provided between the center of the diaphragm200 and the center of the backplate 400 for mechanically suspending andelectrically wiring out the diaphragm 200 using CMOS metalinterconnection method, and the periphery of the diaphragm 200 is freeto vibrate.

This structure typically contains a series of tiny holes or tiny slots,for example, on the edge of diaphragm, in order to control theresistance of air flow in a desired level. This air flow is between thetwo sides of diaphragm and is also called air leakage. When the airleakage rate is too low, the air pressure on the two sides of thediaphragm might be unbalanced. Consequently, a sudden air pressurechange or a sudden acceleration of the microphone may cause a suddenmotion of moving membrane/diaphragm 101, which may damage the delicatemembrane/diaphragm 101. When the air leakage rate is too high, themicrophone may have a descending sensitivity response on low frequencyaudio. Advantageously, the present invention provides a solution to sucha problem with a new design of air flow restrictor having a pair ofcontinuous looped insert and trench, in which the air leakage iscontrolled to a desired range, i.e. not too high and not too low.

SUMMARY OF THE INVENTION

The present invention provides a MEMS device comprising a channel/spacefor air to flow through. The channel/space may be defined by at least afirst internal wall and a second internal wall, and air flows betweenthe two walls. While a continuous (unbroken) looped trench (hereinafter“trench”) is recessed from the first internal wall, a continuous(unbroken) looped insert (hereinafter “insert”) is extended from thesecond internal wall and inserted into the trench. The insert and thetrench have a relative spatial relationship therebetween, and thespatial relationship varies with a frequency F1, which can be any valuegreater than 0.

The above features and advantages and other features and advantages ofthe present invention are readily apparent from the following detaileddescription of the best modes for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements. All the figures areschematic and generally only show parts which are necessary in order toelucidate the invention. For simplicity and clarity of illustration,elements shown in the figures and discussed below have not necessarilybeen drawn to scale. Well-known structures and devices are shown insimplified form in order to avoid unnecessarily obscuring the presentinvention. Other parts may be omitted or merely suggested.

FIG. 1A schematically illustrates a conventional capacitive microphonein the prior art.

FIG. 1B shows a conventional capacitive microphone with a perforatedbackplate in the prior art.

FIG. 1C is a perspective view of a MEMS device with a pair of continuouslooped insert and trench in accordance with an exemplary embodiment ofthe present invention.

FIG. 1D is a cross-sectional view of a MEMS device with a pair ofcontinuous looped insert and trench in accordance with an exemplaryembodiment of the present invention.

FIG. 1E is a cross-sectional view of an internal wall in a MEMS devicewith a pair of continuous looped insert and trench in accordance with anexemplary embodiment of the present invention.

FIG. 2A schematically shows a lateral mode capacitive microphone inaccordance with an exemplary embodiment of the present invention.

FIG. 2B illustrates a lateral mode capacitive microphone in accordancewith an exemplary embodiment of the present invention.

FIG. 3 illustrates acoustic pressures impacting a microphone along arange of directions.

FIG. 4 illustrates the methodology on how to determine the primarydirection for the internal components in a microphone in accordance withan exemplary embodiment of the present invention.

FIG. 5 schematically shows a MEMS capacitive microphone in accordancewith an exemplary embodiment of the present invention.

FIG. 6 illustrates the first/second electrical conductors having a combfinger configuration in accordance with an exemplary embodiment of thepresent invention.

FIG. 7 depicts the spatial relationship between two comb fingers of FIG.6 in accordance with an exemplary embodiment of the present invention.

FIG. 8 shows that four movable membranes are arranged in a 2×2 arrayconfiguration in accordance with an exemplary embodiment of the presentinvention.

FIG. 9 demonstrates the design of one or more such as two air flowrestrictors in accordance with an exemplary embodiment of the presentinvention.

FIG. 10 shows that microphone sensitivity drops at low frequency due toair leakage.

FIG. 11 shows the frequency response with air leakage reduced/preventedin accordance with an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It is apparent, however, to oneskilled in the art that the present invention may be practiced withoutthese specific details or with an equivalent arrangement.

Where a numerical range is disclosed herein, unless otherwise specified,such range is continuous, inclusive of both the minimum and maximumvalues of the range as well as every value between such minimum andmaximum values. Still further, where a range refers to integers, onlythe integers from the minimum value to and including the maximum valueof such range are included. In addition, where multiple ranges areprovided to describe a feature or characteristic, such ranges can becombined.

With reference to FIGS. 1C, 1D and 1E, a MEMS device 12 includes achannel/space 121 defined by a first internal wall 122 and a secondinternal wall 123. In preferred embodiments, the two walls (122, 123)are in parallel with each other. A continuous looped trench (122T) isrecessed into the first internal wall (122), a continuous looped insert(123S) is extended from the second internal wall (123), and thecontinuous looped insert (123S) is inserted into the continuous loopedtrench (122T) (“paired”). In various embodiments of the invention, theinsert (123S) and the trench (122T) are so configured that an “exhaling”scenario and an “inhaling” scenario would or could occur. The “exhaling”scenario may occur if and when the two walls (122, 123) are pushedtoward each other. In the “exhaling” scenario, air within thechannel/space (121) would flow radially or outwardly toward the trench(122T), enters the trench (122T), flows around the insert (123S), andexits out from the outer side of trench (122T) releasing into the outerportion of the channel/space 121 and/or a space outside thechannel/space (121) (e g ambient air). The “inhaling” scenario works inan opposite way, and it may occur if and when the two walls (122, 123)are pulled away from each other. In the “inhaling” scenario, air outsidethe channel/space (121) (e.g. ambient air) would flow inwardly towardthe trench (122T), enters the trench (122T), flows around the insert(123S), exits out from the inner side of trench (122T), and at lastenters the inner portion of the channel/space 121.

A continuous (unbroken) looped trench 122T is recessed into the firstinternal wall 122. A continuous (unbroken) looped insert 123S isextended from the second internal wall 123 and the insert 123S encirclesor surrounds a central region 121C. In an exemplary “exhaling” scenarioas shown in FIG. 1D, when the two walls (122, 123) are pushed towardeach other, air (shown as arrows) within the channel/space 121 flowsalong directions radial from the central region (121C) of thechannel/space (121). The “exhaling” scenario is the opposite of that asshown in FIG. 1D and will be omitted for conciseness. It should beappreciated that trench 122T encircles the central region 121C (or moreprecisely, a portion of the body of wall 122 beneath central region121C). Air resistance of the channel/space 121 may be controlled by thedepth of trench 122T. The air resistance is higher with a deeper trench122T. In preferred embodiments, both walls 122 and 123 have a flatsurface, trench 122T is perpendicular to the flat surface of the firstinternal wall 122, and insert 123S is perpendicular to the flat surfaceof the second internal wall 123. In some embodiments, MEMS device 12 mayinclude, or may not include (is free of), any non-looped or discretetrench/insert (i.e. trench/insert with at least two terminal ends) MEMSdevice 12 may include, or may not include (is free of), any two or morenon-looped or discrete trenches/inserts that are (or not) in parallelwith each other.

In various embodiments of the invention, one or two of the walls 122 and123 may be airtight or ventilated (with one or more through-wall holes).One or two of the walls 122 and 123 may include a single layer ormultiple layers (e.g. laminated). One or two of the walls 122 and 123may be even and flat, or may be irregular and uneven. In an embodimentas shown in FIG. 1E, wall 122 may include multiple layers 122 a, 122 band 122 c. Layer 122 a may be for example a substrate layer, layer 122 bmay be for example an adhesive layer, and layer 122 c may be for examplea PCB plate. Optionally, there may be one or more vents or holes throughwall 122, for example hole 122 h through layer 122 c.

Referring again to FIGS. 1C, 1D and 1E, the insert 123S and the trench122T may have a first relative spatial relationship (SR1) therebetween,which can vary or oscillate or fluctuates with a frequency F1 that canbe zero or any value greater than zero, e.g. when the MEMS device (12)is in a working or operating state. FIG. 1D shows that the insert 123Sand the trench 122T move toward, and away from, each other, in anexaggerated way for a microphone. The continuous looped insert 123S canbe inserted into the continuous looped trench 122T (but it does notcompletely fill the trench 122T so that air can still flow between 122Tand 123S), pulled away from the trench 122T, inserted again, pulled awayagain, and so on and on. In some embodiments, a first mutual capacitance(MC1) can exist between the insert 123S and the trench 122T, and thefirst mutual capacitance (MC1) varies (or fluctuates or oscillates) aswell, for example, varies (or fluctuates or oscillates) in a frequencyF2 that can be any value greater than zero. In preferred embodiments, F1and F2 are independently of each other in the range of from 20 Hz to20,000 Hz, when MEMS device 12 such as a microphone is inworking/operating status or state. In a more preferred embodiment,F1=F2.

In some embodiments, the first internal wall 122 is at least partiallymade of a substrate, or it is a part of a substrate, and the substratemay be for example a substrate for a semiconductor device or a MEMSdevice. The second internal wall 123 may be a movable membrane 123M. Theinsert 123S moves along with the movable membrane 123M when the movablemembrane 123M moves. In preferred embodiments, the MEMS device 12 is acapacitive MEMS microphone 12M. The microphone 12M is configured todetect acoustic wave with frequency F3. For example, the sound wave maycause a variation (or fluctuation or oscillation) of both the relativespatial relationship (SR1) and the mutual capacitance (MC1) between theinsert 123S and the trench 122T, in a manner that F1=F2=F3.

In a variety of exemplary embodiments, MEMS device 12 as shown in FIGS.1C, 1D and 1E may be a capacitive microphone that includes a firstelectrical conductor and a second electrical conductor. The twoconductors are configured to have a second relative spatial relationship(SR2) therebetween so that a second mutual capacitance (MC2) can existbetween them. The relative spatial relationship (SR2) as well as themutual capacitance (MC2) can both be varied or oscillated by an acousticpressure impacting upon the first electrical conductor and/or the secondelectrical conductor along a range of impacting directions in 3D space.Given the same strength/intensity of acoustic pressure, the mutualcapacitance (MC2) can be varied or oscillated the most (or maximallyvaried/oscillated) by an acoustic pressure impacting upon the firstelectrical conductor and/or the second electrical conductor along onedirection among the above range of impacting directions. Such adirection is defined as the primary direction. The first electricalconductor has a first projection along the primary direction on aconceptual plane that is perpendicular to the primary direction. Thesecond electrical conductor has a second projection along the primarydirection on the conceptual plane. The first projection and the secondprojection have a shortest distance Dmin therebetween, and Dmin remainsgreater than zero regardless the first electrical conductor and/or thesecond electrical conductor is (are) impacted by an acoustic pressurealong the primary direction or not. In an embodiment,

With reference to FIG. 2A for more details. In a capacitive microphone200 such as a MEMS microphone, a first electrical conductor 201 and asecond electrical conductor 202 are configured to have a relativespatial relationship (SR2) therebetween so that a mutual capacitance(MC2) can exist between them. The movable membrane 123M may constituteat least a part of the second electrical conductor 202 (including theentire second electrical conductor 202). The first electrical conductor201 and the second electrical conductor 202 are independently of eachother made of polysilicon, gold, silver, nickel, aluminum, copper,chromium, titanium, tungsten, and platinum. The relative spatialrelationship (SR2) as well as the mutual capacitance (MC2) can both bevaried or oscillated by an acoustic pressure impacting upon the firstelectrical conductor 201 and/or the second electrical conductor 202. Asshown in FIG. 3, the acoustic pressure may impact 201 and/or 202 along arange of impacting directions in 3D space as represented by dottedlines. Given the same, strength/intensity of acoustic pressure, themutual capacitance (MC2) can be varied/oscillated the most (or maximallyvaried) by an acoustic pressure impacting upon the first electricalconductor 201 and/or the second electrical conductor 202 along a certaindirection among the above range of impacting directions as shown in FIG.3. The variation of the second mutual capacitance (ΔMC or ΔMC2) causedby various impacting directions of acoustic pressure from 3D space withsame intensity (IDAPWSI) is conceptually plotted in FIG. 4. A primarydirection is defined as the impacting direction that generates the peakvalue of ΔMC (or ΔMC2), and is labeled as direction 210 in FIG. 2A. Itshould be appreciated that, given the same strength/intensity ofacoustic pressure, the relative spatial relationship (SR2) can be variedthe most (or maximally varied) by an acoustic pressure impacting uponthe first electrical conductor 201 and/or the second electricalconductor 202 along a certain direction X among the range of impactingdirections as shown in FIG. 3. Direction X may be the same as, ordifferent from, the primary direction 210 as defined above. In someembodiments of the invention, the primary direction may be alternativelydefined as the direction X.

Referring back to FIG. 2A, the first electrical conductor 201 has afirst projection 201P along the primary direction 210 on a conceptualplane 220 that is perpendicular to the primary direction 210. The secondelectrical conductor 202 has a second projection 202P along the primarydirection 210 on the conceptual plan 220 e. The first projection 201Pand the second projection 202P have a shortest distance Dmintherebetween. In the present invention, Dmin may be constant orvariable, but it is always greater than zero, no matter the firstelectrical conductor 201 and/or the second electrical conductor 202 is(are) impacted by an acoustic pressure along the primary direction 210or not. FIG. 2B illustrates an exemplary embodiment of the microphone ofFIG. 2A. First electrical conductor 201 is stationary, and has afunction similar to the fixed backplate in the prior art. A large flatarea of second electrical conductor 202 including membrane 123M as shownin FIG. 1C and FIG. 1D, similar to movable/deflectablemembrane/diaphragm 102 in FIG. 1A, receives acoustic pressure and movesup and down along the primary direction, which is perpendicular to theflat area. In an embodiment, the entire second electrical conductor 202or the entire membrane 123M (including the central part thereof) movesup along the primary direction or the normal direction of membrane 123M,and then the entire second electrical conductor 202 or the entiremembrane 123M (including the central part thereof) moves down along theprimary direction or the normal direction of membrane 123M, in arepeated manner. However, conductors 201 and 202 are configured in aside-by-side spatial relationship. As one “plate” of the capacitor,second electrical conductor 202 does not move toward and from firstconductor 201. Instead, second conductor 202 laterally moves over, or“glides” over, first conductor 201, producing changes in the overlappedarea between 201 and 202, and therefore varying the mutual capacitance(MC2) therebetween. A capacitive microphone based on such a relativemovement between conductors 201 and 202 is called lateral modecapacitive microphone in the present invention.

In exemplary embodiments of the invention, the microphone may be a MEMS(microelectromechanical System) microphone, AKA chip/silicon microphone.Typically, a pressure-sensitive diaphragm is etched directly into asilicon wafer by MEMS processing techniques, and is usually accompaniedwith an integrated preamplifier. For a digital MEMS microphone, it mayinclude built in analog-to-digital converter (ADC) circuits on the sameCMOS chip making the chip a digital microphone and so more readilyintegrated with digital products.

In an embodiment as shown in FIG. 5, capacitive microphone 200 mayinclude a substrate 230 such as silicon, and first internal wall 122 inFIGS. 1C, 1D and 1E is at least partially made of the substrate, or itis a part of the substrate 230. The substrate 230 can be viewed as theconceptual plane 220 in FIG. 2A. The first electrical conductor 201 andthe second electrical conductor 202 may be constructed above thesubstrate 230 side-by-side. Alternatively, first electrical conductor201 may be surrounding the second electrical conductor 202, as shown inFIG. 5. In an exemplary embodiment, first electrical conductor 201 isfixed relative to the substrate 230. On the other hand, secondelectrical conductor 202 may be a membrane 123M (or includes a membrane123M) that is movable relative to the substrate 230. The primarydirection may be (is) perpendicular to the membrane plane 202. Themovable membrane 202/123M may be attached to the substrate 230 via threeor more suspensions 202S such as four suspensions 202S. As will bedescribed and illustrated later, each of the suspensions 202S maycomprise folded and symmetrical cantilevers.

In an embodiment as shown in FIG. 6, the first electrical conductor 201comprises a first set of comb fingers 201 f. The movable membrane 123M(second conductor 202) comprises a second set of comb fingers 202 faround the peripheral region of the membrane. The two sets of combfingers 201 f and 202 f are interleaved into each other. The second setof comb fingers 202 f are movable along the primary direction, which isperpendicular to the membrane plane 202, relative to the first set ofcomb fingers 201 f. As such, the resistance from air located within thegap between the membrane 202 and the substrate is lowered, for example,25 times lower squeeze film damping. In a preferred embodiment, thefirst set of comb fingers 201 f and the second set of comb fingers 202 fhave identical shape and dimension. As shown in FIG. 7, each comb fingerhas a same width W measured along the primary direction 210, and thefirst set of comb fingers 201 f and the second set of comb fingers 202 fmay have a positional shift PS (or stationary positional shift PS) alongthe primary direction 210, in the absence of any vibration caused bysound wave. For example, the positional shift PS along the primarydirection 210 may be one third (⅓) of the width W, PS=⅓ W. In otherwords, the first set of comb fingers 201 f and the second set of combfingers 202 f have an overlap of ⅔ W along the primary direction 210, inthe absence of any vibration caused by sound wave.

In embodiments, the movable membrane 202/123M may have a shape ofsquare. As shown in FIG. 8, the capacitive microphone of the inventionmay include one or more movable membranes. For example, four movablemembranes can be arranged in a 2×2 array configuration.

In some embodiments as shown in FIG. 9, the capacitive microphone of theinvention comprises one or more such as two air flow restrictors 241that restrict the flow rate of air that flows in/out of the gap betweenthe membrane 202/123M and the substrate 230 (or first internal wall 122as shown in FIGS. 1C, 1D and 1E). Air flow restrictors 241 may bedesigned to decrease the cross section area (size) of an air channel 240(or channel/space 121 as shown in FIG. 1C and FIG. 1D) for the air toflow in/out of the gap, as compared to a capacitive microphone withoutsuch air flow restrictor 241. Alternatively or additionally, air flowrestrictors 241 may increase the length of the air channel 240 (orchannel/space 121 as shown in FIG. 1C and FIG. 1D) for the air to flowin/out of the gap, as compared to a capacitive microphone without suchair flow restrictor 241. For example, air flow restrictors 241 maycomprise an insert 242 (an example of 123S in FIG. 1C and FIG. 1D) intoa groove 243 (an example of 122T in FIGS. 1C, 1D and 1E), which not onlydecreases the cross section area of an air channel 240 (or channel/space121 as shown in FIG. 1C and FIG. 1D), but also increases the length ofthe air channel 240 (or channel/space 121 as shown in FIG. 1C and FIG.1D).

Referring back to FIGS. 6 and 7, comb fingers 201 f are fixed on anchor,and comb fingers 202 f are integrated with membrane-shaped secondelectrical conductor 202 (hereinafter membrane 202 or membrane 202/123M,for simplicity). When membrane 202/123M vibrates due to sound wave,fingers 202 f move together with membrane 202/123M. The overlap areabetween two neighboring fingers 201 f and 202 f changes along with thismovement, so does the capacitance MC2. Eventually a capacitance changesignal is detected that is the same as conventional capacitivemicrophone.

Leakage is always a critical issue in microphone design. In conventionalparallel plate design as shown in FIG. 1A, it typically has a couple oftiny holes around the edge in order to let air go through slowly, tokeep air pressure balance on both sides of membrane 101 when itexperiences undesired vibration or deflection, for example with afrequency of less than 20 Hz. That is a desired leakage. However, alarge leakage is undesired, because it will let some low frequency soundwave escape away from membrane vibration easily via the holes, and willresult in a sensitivity drop in low frequency, for example around 100Hz. FIG. 10 shows that sensitivity drops at low frequency due toleakage. For a typical capacitive MEMS microphone, the frequency rangeis between 20 Hz and 20 kHz, thus the sensitivity drop in FIG. 10 isundesired.

In order to prevent this large leakage, a more preferred structure isdesigned and shown in FIG. 9, which illustrates a leakage prevent grooveor slot and wall. Referring back to FIG. 9, air flow restrictors 241 mayfunction as a structure for preventing air leakage in the microphone ofthe invention. Air flow restrictor 241 comprises an insert 242 into agroove 243, which not only decreases the cross section area (size) of anair channel 240, but also increases the length of the air channel 240.In MEMS microphones, a deep slot may be etched on substrate around theedge of square membrane 202 and then a wall 242 connected to membrane202 is deposited to form a long and narrow air tube 240, which gives alarge acoustic resistance. FIG. 11 depicts the frequency response withleakage prevented. This leakage prevention structure has a significanteffect on keeping the frequency response plot more flat on the range 100Hz to 1000 Hz. The level of the air resistance may be controlled by theslot depth etched on the substrate. The deeper slot, the higher theresistance.

Applicant's co-pending U.S. application Ser. No. 15/730,732 filed onOct. 12, 2017 teaches a process of fabricating a capacitive microphonesuch as a MEMS microphone of the present invention. In the process, oneelectrically conductive layer is deposited on a removable layer, andthen divided or cut into two divided layers, both of which remain incontact with the removable layer as they were. One of the two dividedlayers will become or include a movable or deflectablemembrane/diaphragm that moves in a lateral manner relative to anotherlayer, instead of moving toward/from another layer. The entire contentof U.S. application Ser. No. 15/730,732 is incorporated herein byreference.

In the foregoing specification, embodiments of the present inventionhave been described with reference to numerous specific details that mayvary from implementation to implementation. The specification anddrawings are, accordingly, to be regarded in an illustrative rather thana restrictive sense. The sole and exclusive indicator of the scope ofthe invention, and what is intended by the applicant to be the scope ofthe invention, is the literal and equivalent scope of the set of claimsthat issue from this application, in the specific form in which suchclaims issue, including any subsequent correction.

The invention claimed is:
 1. A MEMS device comprising a channel/spacedefined by a first internal wall and a second internal wall that is inparallel with the first internal wall, wherein a continuous loopedtrench is recessed into the first internal wall, wherein a continuouslooped insert is extended from the second internal wall, wherein thecontinuous looped insert is inserted into the continuous looped trench,wherein the insert and the trench have a first relative spatialrelationship (SR1) therebetween, wherein the spatial relationship (SR1)varies or oscillates with a frequency F1≥0, when the MEMS device is in aworking or operating state, wherein said first wall is a part of asubstrate; said second wall is a movable membrane, or a part of amovable membrane, or connected to a movable membrane; wherein the MEMSdevice comprises a first electrical conductor, and the movable membraneconstitutes at least a part of a second electrical conductor, orstructurally connected to a second electrical conductor, wherein thefirst electrical conductor and the second electrical conductor areconfigured to have a relative spatial relationship therebetween, whereina mutual capacitance exists between the first electrical conductor andthe second electrical conductor; wherein said relative spatialrelationship and said mutual capacitance can both be varied by anacoustic pressure impacting upon the first electrical conductor and/orthe second electrical conductor along a range of impacting directions in3D space; wherein said mutual capacitance is varied the most by anacoustic pressure impacting upon the first electrical conductor and/orthe second electrical conductor along one direction among said range ofimpacting directions, said one direction being defined as the primarydirection; wherein the first electrical conductor has a first projectionalong said primary direction on a conceptual plane that is perpendicularto said primary direction; wherein the second electrical conductor has asecond projection along said primary direction on the conceptual plane;and wherein the first projection and the second projection have ashortest distance Dmin therebetween, and Dmin remains greater than zeroregardless of that the first electrical conductor and/or the secondelectrical conductor is (are) impacted by an acoustic pressure alongsaid primary direction or not.
 2. The MEMS device according to claim 1,wherein a mutual capacitance exists between said insert and said trench,and said mutual capacitance between said insert and said trench variesor oscillates with a frequency F2 when the MEMS device is in a workingor operating state, and F1=F2.
 3. The MEMS device according to claim 2,wherein F1 and F2 are in the range of from 20 Hz to 20,000 Hz, the rangeof audible frequencies for humans.
 4. The MEMS device according to claim1, which is a capacitive MEMS microphone.
 5. The MEMS device accordingto claim 4, wherein the microphone is configured to detect sound withfrequency F3, and F1=F2=F3, when the microphone is in a working oroperating state.
 6. The MEMS device according to claim 1, wherein thefirst electrical conductor and the second electrical conductor areindependently of each other made of polysilicon, gold, silver, nickel,aluminum, copper, chromium, titanium, tungsten, or platinum.
 7. The MEMSdevice according to claim 1, wherein the substrate is flat and can beviewed as said conceptual plane, and wherein the first electricalconductor and the second electrical conductor are constructed above thesubstrate side-by-side.
 8. The MEMS device according to claim 7, whereinthe first electrical conductor is fixed relative to the substrate,wherein the movable membrane is movable relative to the substrate, andwherein said primary direction is perpendicular to the membrane plane.9. The MEMS device according to claim 8, wherein the movable membrane isattached to the substrate via three or more suspensions such as foursuspensions.
 10. The MEMS device according to claim 9, wherein thesuspension comprises folded and symmetrical cantilevers.
 11. The MEMSdevice according to claim 8, wherein the first electrical conductorcomprises a first set of comb fingers, wherein the movable membranecomprises a second set of comb fingers around the peripheral region ofthe membrane, and wherein the two sets of comb fingers are interleavedinto each other.
 12. The MEMS device according to claim 11, wherein thesecond set of comb fingers is laterally movable relative to the firstset of comb fingers.
 13. The MEMS device according to claim 11, whereinthe first set of comb fingers and the second set of comb fingers haveidentical shape and dimension.
 14. The MEMS device according to claim13, wherein each comb finger has a same width measured along the primarydirection; and the first set of comb fingers and the second set of combfingers have a positional shift along the primary direction.
 15. TheMEMS device according to claim 8, wherein the movable membrane is squareshaped.
 16. The MEMS device according to claim 15, which comprises oneor more of said movable membranes, such as four movable membranesarranged in a 2×2 array configuration.
 17. The MEMS device according toclaim 8, wherein the insert and the trench increase the length of an airchannel for the air to flow in/out of the gap or channel/space betweenthe membrane and the substrate.
 18. The MEMS device according to claim8, further comprising one, two or more pairs of continuous looped trenchand continuous looped insert.